Process of producing nitrocellulose



Feb. 27, 1934. M, O SCHUR 1,948,944

PROCESS OF PRODUCING NITROCELLULOSE Filed July 17, 1930 (f/20672215 fZa(73 (P672447 iii/ Patented eb. 27, 1934 EQE PROCESS OF PRODUCINGNITROCELLULOSE Milton 0. Schur, Berlin, N. H., assignor to BrownCompany, Berlin, N. H., a corporation of Maine Application July 17,1930. Serial No. 468,529

Claims.

This invention relates to a process of producing nitrocelluloses, and,while not restricted thereto, it is concerned with a process in which acompact or dense form of cellulose is used as a raw I material.

In the nitrocellulose industry, it has heretofore been the generalcustom to use cellulose in bulky form as a raw material, for instance aslinters, shredded paper, pulp, etc. Such raw material re- 0 quires alarge amount of nitrating acid in order to be properly nitrated. Infact, the ratio of nitrating acid to dry cellulose required is usually aminimum of about fifty to one. This means that a large excess ofnitrating acid is constantly 16 tied up in the process, and further thatthis acid must be handled along with the cellulose as a bulky mass afternitration has been completed, that is, up to the point where the excessacid is separated out as by centrifugation. The large 20 excess of acidpresent during the nitrating operation, however, ensures a nitratingacid of practically fixed composition, since the generation of waterduring nitration and the consumption of nitric acid effects but anegligible weakening of the acid.

In accordance with the present invention, I preferably use a compact ordense form of cellulose as a raw material for making nitrocelluloses.More particularly, I prefer to use a nitration grade of wood pulp in theform of comparatively thick sheets or boards known, in the trade asdrier sheets. These sheets are supplied by pulp manufacturers, who findit economical practice to run off their processed pulps on pulp drierswhich may be built as large units capable of handling 100 or more tonsof pulp per day. Various types of wood pulp are sold in such form,- forinstance, the sulphite pulp used by viscoserayon manufacturers. Fornitration purposes,

however, the wood pulp should preferably be a pure white, refinedproduct of high alpha cellulose content, low soda solubility, and othercharacteristics specified by nitrocellulose converters.

The sheets, which may be of a thickness of, say,

0.020 to 0.050 inches, are compact or dense, as

compared with cotton linters, because of the interfelted relation of thefibers. When such sheets are nitrated by steeping in a bath of nitratingacid, a much lower ratio of nitrating acid to dry fiber is necessary toensure complete submergence of the sheets than that necessary in thecase of a bulky material such as cotton linters, but if this ratio istoo low, the acid is so weakened as nitration proceeds that it will failto nitrate properly. In accordance with the present invenk, tion, acomparatively low acid to fiber ratio is employed in the nitrating vat,for instance, less than about forty to one, but as nitration proceeds,acid of the desired composition is progressively fed into the vat whilethe acid is being progressively withdrawn from the vat, preferably at apoint remote from the point of feeding, for fortification and reuse, sothat there is a constant circulation of acid of the desired compositionin the vat to ensure proper nitration. Whereas circulation of acidthrough a bulky mass is not entirely practicable, with dense sheets itis easy to afford such circulation while a bath of comparatively lowacid to fiber ratio is being maintained in the nitrating vat. Such aprocedure is advantageous, in that it permits a high nitrocelluloseproduction with comparatively little nitrating acid being tied up in theprocess, as I may provide a master pool or reservoir of excess acid ofthe proper composition for a large number of nitrating vats or units.Acid may be constantly supplied in regulated amount from the master poolto each of the nitrating units While acid is being constantly withdrawnfrom I the nitrating units, fortified, and returned to the reservoir.Such a procedure is further advantageous, in that the nitrating unitsmay be of small size because of the compactness of the cellulose and thecomparatively small volume of acid present therein during the nitratingoperation.

A further advantage of my process isthat the temperature of all thebaths of acid may be easily maintained substantially constant throughoutthe nitrating acid, since it is necessary to control only thetemperature of the master poolof acid for this purpose. As is well knownto those skilled in the art, the quality or grade of nitrocellulose isaifected not only by the composition of the nitrating acid, but also bythe temperature maintained during the nitrating operation. By applyingmy invention, therefore, it ispossible to regulate closely the qualityof the nitrocellulose produced.

The master pool may be of sufiicient volume so that it is unnecessary tofortify the acid being withdrawn from the nitrating units and returnedto the pool, particularly if the acid is circulated through the baths ata rate ensuring the recirculation of the total quantity of nitratingacid from the master pool through the baths a plurality of times duringa nitrating operation, as in such case the acid in the baths ismaintained substantially fixed in composition. It is, however,preferable to fortify the acid being withdrawn from the baths before itis returned to the master pool, as in such case the total quantity ofacid in the system may be far less than that necessary when a bulky formof cellulose such as cotton linters is employed as a raw material. Inany event, however, the system is preferably operated so that the totalvolume of acid being used therein makes a number of circuits through thenitrating units, as such mode of operation makes possible nitrationunder substantially fixed conditions of both acid. composition andtemperature in the baths.

With these and other objects and features in view, my invention will nowbe described with greater particularity in conjunction with theaccompanying drawing, which illustrates more or less diagrammaticallyand conventionally an apparatus embodying the principles of theinvention.

Referring to the drawing, 1, 2 and 3 represent a series of closednitrating vats or units in which the nitration of sheets of cellulose ofthe kind described may be carried out. The walls of vat 1 are shownpartly broken away, to illustrate how sheets 4 may be arranged insidethe vat in closely spaced, parallel relation, as by suspension from rods5. Nitrating acid of the desired composition may be supplied to the vatsin amount sufhcient to submerge the sheets from a storage tank 6 shownlocated above the vats and in which a master pool of nitrating acid maybe kept for the nitrating baths. A coil 7, through which a heating orcooling medium may be passed, is shown within the tank 6, so that themaster pool may be maintained at the desired temperature. The acid maybe allowed to gravitate from the tank 6 through a pipe 8 into a manifold9 which leads the acid through a series of valved connections 11, eachof which enters into a vat through its end 10, preferably near itsbottom. Communicating by means of valved connections 25 with the vatsthrough their other ends 12, preferably near their tops, is a manifold13, which is connected by a pipe 14 to a pump 15 by which the acid maybe withdrawn from the vat and fed through a pipe 16 into a chamber 17,shown above the vats. Preferably also a series of valved bottomconnections 26 are provided between the vats and the manifold 13, whichwill permit removing all the acid from the vats by the pump 15 when thenitrating operation has been completed. By feeding acid into each vat atone end near its bottom and withdrawing it at the other end near itstop, a positive flow and diffusion of acid of the desired composition isensured through each vatand the accumulation of weakened acid at anypoint in the bath is avoided.

The acid fed into the chamber 17 may be fortified or strengthened to thedesired composition by delivering fortifying acid in regulatablequantities thereinto as from a doser 18 located above the chamber andsupplied with acid by gravity from a storage tank 19 located thereabove,in which a supply of the fortifying acid is kept. A stirrer 20 ispreferably provided in the chamber 17 to effect a uniform mixture of theacids. The

fortified acid may gravitate from the chamber 17 through a pipe 21,valved as at 22, into a pump 23 which discharges into the top of thetank '7 through a pipe 24.

When nitration is going on, there is a constant circulation of acid inthe system from the stor-- age tank 7 through the nitrating vats,through the mixing chamber 1'7, back to the storage tank '7, the acidwithdrawn from the vats being dosed in the chamber 17 with theappropriate amount of fortifying acid to restore it to the desiredcondition. This circulation is preferably carried on at such a rate thatthe total volume of acid in the system completes a plurality of circuitsduring a single nitrating operation, so as to ensure practicallyconstant nitrating conditions in each nitrating vat. In operating theapparatus described it is unnecessary to start all the vats at once. Infact, it may be desirable that charge of cellulose at various stages ofreaction are in process in the vats at any one time, as this would notonly tend to keep the composition of the acid circulation through thesystem more nearly uniform, but would also tie in better with theoperations of charging, washing, drowning, stabilizing, etc.

The principles of the present invention may be applied when the sheetsare cut into small pieces, for by providing strainers or screens at thepoints of communication of the manifolds 9 and 13 with the vats, thepieces may be kept in the vats, but in such case stirrers are preferablyalso provided in the vats, so as to offset the tendency for the piecesto accumulate or pack at the regions where acid is being withdrawn.Ordinarily, however, it is preferable to use sheets of comparativelylarge size suspended from rods or held in suitable clamps or frames.They are maintained entirely submerged in the nitrating acid, so thatthe nitrating acid may contact uniformly with all the surface presentedand effect a uniform nitration. Such large sheets may be handled readilyas such through the after-nitration treatments, and constitute aneconomical and convenient form in which to store, pack, and ship thefinished product.

I claim:

1. A process which comprises steeping compact sheets of cellulose as astationary batch in a bath of nitrating acid of the desired composition,and progressively feeding acid of said desired composition into the bathwhile progressively withdrawing acid from the bath as nitrationproceeds.

2. A process which comprises steeping compact sheets of cellulose as astationary batch in a bath of nitrating acid of the desired composition,and as nitration proceeds progressively introducing acid of said desiredcomposition into the bath while progressively withdrawing acid from thebath at a point remote from the point of introduction of acid.

3. A process which comprises steeping compact sheets of cellulose as astationary batch in a bath of nitrating acid of the desired composition,progressively feeding acid of said desired composition into the bathwhile progressively withdrawing acid from the bath as nitrationproceeds, fortifying said withdrawn acid to said desired composition,and using it as the acid for feeding into the bath.

4:. A process which comprises steeping compact sheets of cellulose as astationary batch in a bath of nitrating acid of the desired composition,maintaining an independent pool of acid of said composition,progressively feeding acid from saidpool into said bath whileprogressively withdrawing acid from said bath, fortifying said withdrawnacid to the desired composition, and delivering said fortified acid intosaid pool.

5. A process which comprises steeping compact 5 sheets of cellulose asstationary batches in a plurality of baths of nitrating acid of thedesired composition, maintaining a master pool of acid of said desiredcomposition for said baths, progressively feeding acid from said poolinto said baths while progressively withdrawing acid from said baths,fortifying said withdrawn acid to the desired composition, anddelivering said fortified acid into said pool.

6. A process of nitrating compact sheets of cellulose as stationarybatches in a plurality of nitrating baths, which comprises maintaining amaster pool of nitrating acid in which said sheets are submersed incirculating communication with each of the baths, and while nitration isin progress causing progressive circulation of the acid from the masterpool through the baths at a rate ensuring the recirculation of the totalquantity of acid a plurality of times.

7. A process which comprises nitrating compact sheets of cellulose as astationary batch in which the ratio of nitrating acid to such cellulosein the reaction vessel is less than about 40 to 1 throughout thenitrating reaction and in which the nitrating acid is fortified duringthe course of the reaction on such cellulose.

8. A process which comprises nitrating compact sheets of cellulose as astationary batch in which the ratio of nitrating acid to such cellulosein the reaction is initially less than 40 to 1, and maintaining thecomposition of the nitrating acid and its ratio to the cellulosesubstantially constant until nitration has been completed.

9. A process which comprises nitrating compact sheets of cellulose as astationary batch in a bath of nitrating acid, and uniformly andprogressively fortifying the nitrating acid in said bath to maintain itscomposition substantially constant throughout the nitrating operation.

10. A process which comprises nitrating compact sheet of cellulose as astationary batch in which the ratio of nitrating acid to cellulose inthe reaction is initially less than 40 to 1, and uniformly andprogressively fortifying the nitrating acid in said batch with freshreagents to maintain the composition of the nitrating acid and its ratioto the cellulose substantially constant until nitration has beencompleted.

MILTON O. SCHUR.

